Orthogonal frequency division multiplexing (OFDM) modems are designed to meet the impairment of various communications environments that vary from a quasi-static channel of a wireline system, to a highly dynamic channel of a fast moving wireless mobile system. While the channel has a large impact on the modem performance and design, other requirements such as data rate, BER and latency of the end-to-end application places additional requirements, impacting the final design of the modem.
The OFDM modems for packet-based applications such as WiFi and WiMax usually can tolerate large latencies and as such can have interleaving, coding and preambles for channel estimation. A number of pilot channels, in the frequency domain, are also used to aid operations such as residual frequency offset removal, clock timing recovery and other fine tuning operations that require symbol-by-symbol correction. In circuit-switched (continues transmission) type OFDM links, most of the synchronization and acquisition algorithms are based on continuous received symbols (decision aided or directed). In CS OFDM links, while the latency requirements may be tighter than a packet-switch OFDM, the modem can still enjoy the presence of pilot tones in frequency, or time domain. The invention described here is to enable an OFDM modem with severe time and/or frequency constraint to perform channel estimation for the functional units in a modem such AFC, Maximal Ratio Combining and MIMO.